FREEZERS WITH CASCADE REFRIGERATION SYSTEMS USING PARALLEL EXPANSION DEVICES FOR ADJUSTABLE EXPANSION

Abstract

A cascade refrigeration system having a first refrigeration stage that defines a first fluid circuit and includes a first expansion device and a second refrigeration stage that defines a second fluid circuit, fluidically isolated from the first fluid circuit, that includes a second expansion device. The second expansion device includes a first capillary tube and a second capillary tube in parallel flow arrangement, and a second stage valve in fluid communication with the second capillary tube for selectively controlling flow of the second refrigerant through the second capillary tube in response to at least one operating condition of the refrigeration system without interrupting flow of the second refrigerant through the first capillary tube. The refrigeration system further includes at least one interstage heat exchanger in heat transferring communication with the first and second fluid circuits to exchange heat between the first and second refrigerants.

Claims

1. A refrigeration system, comprising: a first refrigeration stage defining a first fluid circuit for circulating a first refrigerant, the first refrigeration stage having: a first condenser; a first compressor; and a first expansion device in fluid communication with the first fluid circuit; a second refrigeration stage defining a second fluid circuit that is fluidically isolated from the first fluid circuit for circulating a second refrigerant, the second refrigeration stage having: a second condenser; a second compressor; and a second expansion device in fluid communication with the second fluid circuit, the second expansion device comprising: a first capillary tube and a second capillary tube in parallel flow arrangement; and a second stage valve in fluid communication with the second capillary tube for selectively controlling flow of the second refrigerant through the second capillary tube in response to at least one operating condition of the refrigeration system without interrupting flow of the second refrigerant through the first capillary tube; and at least one interstage heat exchanger in heat transferring communication with the first and second fluid circuits to exchange heat between the first and second refrigerants, wherein the first and second refrigeration stages are configured in a cascade cooling arrangement.

2. The refrigeration system of claim 1, wherein the first fluid circuit further comprises: a first fluid line in fluid communication with an outlet of the first condenser and a first inlet of the at least one interstage heat exchanger, the first fluid line including the first expansion device; a first suction line in fluid communication with a first outlet of the at least one interstage heat exchanger and an inlet of the first compressor; and a first heat exchanger defined by a portion of the first fluid line or a portion of the first expansion device being in heat transferring communication with a portion of the first suction line to thereby exchange heat between the first refrigerant flowing through the first fluid line and the first refrigerant flowing through the first suction line.

3. The refrigeration system of claim 1, wherein the second fluid circuit further comprises: a second fluid line in fluid communication with a second outlet of the at least one interstage heat exchanger and an inlet of the evaporator, the second fluid line including the second expansion device; a second suction line in fluid communication with an outlet of the evaporator and an inlet of the second compressor; and a second heat exchanger defined by a portion of the second fluid line or a portion of the second expansion device being in heat transferring communication with a portion of second suction line to thereby exchange heat between the second refrigerant flowing through the second fluid line and the second refrigerant flowing through the second suction line.

4. The refrigeration system of claim 2, wherein the second fluid circuit further comprises: a second fluid line in fluid communication with a second outlet of the at least one interstage heat exchanger and an inlet of the evaporator, the second fluid line including the second expansion device; a second suction line in fluid communication with an outlet of the evaporator and an inlet of the second compressor; and a second heat exchanger defined by a portion of the second fluid line or a portion of the second expansion device being in heat transferring communication with a portion of second suction line to thereby exchange heat between the second refrigerant flowing through the second fluid line and the second refrigerant flowing through the second suction line.

5. The refrigeration system of claim 1, wherein the first expansion device comprises an electronic expansion valve.

6. The refrigeration system of claim 1, wherein the second stage valve comprises a solenoid valve.

7. The refrigeration system of claim 1, wherein the second stage valve comprises an electronic expansion valve.

8. The refrigeration system of claim 1, wherein the first expansion device further comprises: a third capillary tube and a fourth capillary tube in parallel flow arrangement; and a first stage valve for selectively controlling flow of the first refrigerant through the fourth capillary tube in response to at least one operating condition of the refrigeration system without interrupting flow of the first refrigerant through the third capillary tube.

9. The refrigeration system of claim 8, wherein the first fluid circuit further comprises: a first fluid line in fluid communication with an outlet of the first condenser and a first inlet of the at least one interstage heat exchanger, the first fluid line including the first expansion device; a first suction line in fluid communication with a first outlet of the at least one interstage heat exchanger and an inlet of the first compressor; and a first heat exchanger defined by a portion of the first suction line being in heat transferring communication with a portion of the first fluid line or a portion of the first expansion device to thereby exchange heat between the first refrigerant flowing through the first fluid line and the first refrigerant flowing through the first suction line.

10. The refrigeration system of claim 9, wherein the second fluid circuit further comprises: a second fluid line in fluid communication with a second outlet of the at least one interstage heat exchanger and an inlet of the evaporator, the second fluid line including the second expansion device; a second suction line in fluid communication with an outlet of the evaporator and an inlet of the second compressor; and a second heat exchanger defined by a portion of the second fluid line or a portion of the second expansion device being in heat transferring communication with a portion of second suction line to thereby exchange heat between the second refrigerant flowing through the second fluid line and the second refrigerant flowing through the second suction line.

11. The refrigeration system of claim 8, wherein the first stage valve is a solenoid valve.

12. The refrigeration system of claim 8, wherein the first stage valve is an electronic expansion valve.

13. The refrigeration system of claim 8, wherein the second stage valve is an electronic expansion valve.

14. The refrigeration system of claim 8, wherein the second stage valve comprises a solenoid valve.

15. The refrigeration system of claim 1, wherein the refrigeration system is associated with a cabinet having an interior to be cooled, and the first and second refrigeration stages are operated to cool the cabinet to a temperature setpoint ranging between 20 C. and 90 C.

16. A freezer, comprising: a cabinet having a cabinet interior and a door providing access into the cabinet interior; a cascade refrigeration system, which comprises: a first refrigeration stage defining a first fluid circuit for circulating a first refrigerant, the first refrigeration stage having: a first condenser; a first compressor; and a first expansion device in fluid communication with the first fluid circuit; a second refrigeration stage defining a second fluid circuit that is fluidically isolated from the first fluid circuit for circulating a second refrigerant, the second refrigeration stage having: a second condenser; a second compressor; and a second expansion device in fluid communication with the second fluid circuit, the second expansion device comprising: a first capillary tube and a second capillary tube in parallel flow arrangement; and a second stage valve in fluid communication with the second capillary tube for selectively controlling flow of the second refrigerant through the second capillary tube in response to at least one operating condition of the refrigeration system without interrupting flow of the second refrigerant through the first capillary tube; and at least one interstage heat exchanger in heat transferring communication with the first and second fluid circuits to exchange heat between the first and second refrigerants, wherein the first and second refrigeration stages are configured in a cascade cooling arrangement.

17. The freezer of claim 16, wherein the first fluid circuit further comprises: a first fluid line in fluid communication with an outlet of the first condenser and a first inlet of the at least one interstage heat exchanger, the first fluid line including the first expansion device; a first suction line in fluid communication with a first outlet of the at least one interstage heat exchanger and an inlet of the first compressor; and a first heat exchanger defined by a portion of the first fluid line being in heat transferring communication with a portion of the first suction line or a portion of the first expansion device to thereby exchange heat between the first refrigerant flowing through the first fluid line and the first refrigerant flowing through the first suction line.

18. The freezer of claim 16, wherein the second fluid circuit further comprises: a second fluid line in fluid communication with a second outlet of the at least one interstage heat exchanger and an inlet of the evaporator, the second fluid line including the second expansion device; a second suction line in fluid communication with an outlet of the evaporator and an inlet of the second compressor; and a second heat exchanger defined by a portion of the second fluid line or a portion of the second expansion device being in heat transferring communication with a portion of second suction line to thereby exchange heat between the second refrigerant flowing through the second fluid line and the second refrigerant flowing through the second suction line.

19. The freezer of claim 17, wherein the second fluid circuit further comprises: a second fluid line in fluid communication with a second outlet of the at least one interstage heat exchanger and an inlet of the evaporator, the second fluid line including the second expansion device; a second suction line in fluid communication with an outlet of the evaporator and an inlet of the second compressor; and a second heat exchanger defined by a portion of the second fluid line or the second expansion device being in heat transferring communication with a portion of second suction line to thereby exchange heat between the second refrigerant flowing through the second fluid line and the second refrigerant flowing through the second suction line.

20. The freezer of claim 16, wherein the first expansion device further comprises: a third capillary tube and a fourth capillary tube in parallel flow arrangement; and a first stage valve for selectively controlling flow of the first refrigerant through the fourth capillary tube in response to at least one operating condition of the refrigeration system without interrupting flow of the first refrigerant through the third capillary tube.

21. The freezer of claim 20, wherein the first fluid circuit further comprises: a first fluid line in fluid communication with an outlet of the first condenser and a first inlet of the at least one interstage heat exchanger, the first fluid line including the first expansion device; a first suction line in fluid communication with a first outlet of the at least one interstage heat exchanger and an inlet of the first compressor; and a first heat exchanger defined by a portion of the first suction line being in heat transferring communication with a portion of the first fluid line or a portion of the first expansion device to thereby exchange heat between the first refrigerant flowing through the first fluid line and the first refrigerant flowing through the first suction line.

22. The freezer of claim 21, wherein the second fluid circuit further comprises: a second fluid line in fluid communication with a second outlet of the at least one interstage heat exchanger and an inlet of the evaporator, the second fluid line including the second expansion device; a second suction line in fluid communication with an outlet of the evaporator and an inlet of the second compressor; and a second heat exchanger defined by a portion of the second fluid line or a portion of the second expansion device being in heat transferring communication with a portion of second suction line to thereby exchange heat between the second refrigerant flowing through the second fluid line and the second refrigerant flowing through the second suction line.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, explain the principles of the invention.

[0023] FIG. 1 is a perspective view of a freezer including a cabinet and a deck configured to contain a cascade refrigeration system for cooling the cabinet to ultra-low temperatures, in accordance with one embodiment of the invention.

[0024] FIG. 2 is a schematic representation of an exemplary cascade refrigeration system for cooling the cabinet of the freezer of FIG. 1.

[0025] FIG. 3 is a simplified schematic representation of the primary elements of the first and second refrigeration stages of the cascade refrigeration system of FIG. 2, in accordance with a first embodiment having parallel capillary tubes as expansion devices in one of the refrigeration stages.

[0026] FIG. 4 is a simplified schematic representation of the primary elements of the first and second refrigeration stages of the cascade refrigeration system of FIG. 2, in accordance with a second embodiment having parallel capillary tubes as an expansion device in both refrigeration stages.

DETAILED DESCRIPTION

[0027] With reference to the figures, an exemplary high-performance laboratory freezer 10 according to embodiments of the present invention is illustrated. Although the terms high performance laboratory freezer and freezer are used throughout the specification, it will be understood that the invention encompasses any type of cooling device, including any type of refrigerator or freezer, for example. As shown in FIG. 1, the freezer 10 includes a cabinet 12 for storing items that require cooling to temperatures of about 20 C. or lower, for example. In this regard, the freezer 10 may be considered an Ultra Low Temperature (ULT) freezer designed to operate in a temperature range of about 20 C. to about 90 C., for example. The cabinet 12 includes a cabinet housing 14 defining a generally rectangular cross-section and a door 16 providing access through a cabinet opening into a refrigerated interior 18 of the cabinet 12. The refrigerated cabinet interior 18 is configured to receive products to be maintained at a low temperature such as blood or other biologic samples, or medicines, for example. The cabinet 12 is mounted on top of a deck 20 that is configured to contain the refrigeration system 22 (FIG. 2) of the freezer 10 as set forth in further detail below.

[0028] The exemplary refrigeration system 22 of the present invention includes first and second cascaded refrigeration stages, each including one or more expansion devices that provide adjustable or varying amounts of refrigerant expansion to allow each of the heat exchangers in the refrigeration stages to operate at optimized temperatures when cooling the cabinet interior 18 to a temperature setpoint. As will be described in further detail below, in one embodiment, the second refrigeration stage includes a pair of capillary tubes (for example, a first and a second capillary tube) in parallel flow arrangement and a valve controlling refrigerant flow into one of the capillary tubes to provide the adjustable refrigerant expansion. More particularly, refrigerant flows freely through a first capillary tube while the valve is provided for selectively controlling flow of the refrigerant through a second capillary tube in response to at least one operating condition of the refrigeration system without interrupting flow of the refrigerant through the first capillary tube. Operation of the refrigeration system 22 in this regard provides varying amounts of refrigerant expansion at the very low operating temperatures of the second refrigeration stage in the context of an ULT freezer. Consequently, the freezer 10 of the following embodiments provides optimal performance in varying operational circumstances (including minimized energy consumption and minimized peak temperature variations over time), not just in a worst-case operating condition.

[0029] One example of a detailed schematic layout of the cascade refrigeration system 22 used with embodiments of the freezer 10 of the present invention is shown with reference to FIG. 2. As shown in this schematic layout, the refrigeration system 22 is made up of a first refrigeration stage 24 and a second refrigeration stage 26 respectively defining a first fluid circuit 28 and a second fluid circuit 30. The first and second fluid circuits 28,30 are provided for circulating a first refrigerant 32 and a second refrigerant 34, respectively. The first refrigeration stage 24 transfers energy (for example, heat) from the first refrigerant 32 to the surrounding environment 36, while the second refrigerant 34 of the second refrigeration stage 26 receives thermal energy from the cabinet interior 18. Heat is transferred from the second refrigerant 34 to the first refrigerant 32 through an interstage heat exchanger 38 that is in fluid communication with the first and second refrigeration stages 24, 26 of the refrigeration system 22, with the first and the second fluid circuits 28, 30 being fluidically isolated from each other, but in thermal communication with each other via the interstage heat exchanger 38.

[0030] The first refrigeration stage 24 includes, in sequence, a first compressor 40, a condenser 42, a first expansion device 44, and the interstage heat exchanger 38. A fan 46 directs filtered ambient air across the condenser 42 and facilitates the transfer of heat from the first refrigerant 32 to the surrounding environment 36. The second refrigeration stage 26 includes, also in sequence, a second compressor 48, the interstage heat exchanger 38, a second expansion device 50, and an evaporator 52. The evaporator 52 is in thermal communication with the cabinet interior 18 of cabinet 12 such that heat is transferred from the cabinet interior 18 to the second refrigerant 34 circulating through evaporator 52, thereby cooling the cabinet interior 18. Each of the first and second refrigeration stages 24, 26 may include various other additional components known in the art, such as an oil separator, a de-superheater, a filter drier, and/or a suction accumulator, for example. The functionality of these various additional elements in the refrigeration system 22 will be well understood by those skilled in the refrigeration art and are therefore not described.

[0031] Referring to FIG. 2, in operation, the second refrigerant 34 receives heat from the cabinet interior 18 through the evaporator 52 and flows from the evaporator 52 to the second compressor 48, as indicated by the directional arrows. From the second compressor 48, the compressed second refrigerant 34 flows into the interstage heat exchanger 38 thermally communicating the first and second refrigeration stages 24, 26 with one another. The second refrigerant 34 enters the interstage heat exchanger 38 in gas form and transfers heat to the first refrigerant 32 as the second refrigerant 34 condenses. In this regard, the flow of the first refrigerant 32 may, for example, be counter-flow relative to the second refrigerant 34, to maximize the rate of heat transfer. In one specific, non-limiting example, the heat exchanger 38 is in the form of a brazed plate heat exchanger, vertically oriented within the deck 20 of the freezer 10 and designed to maximize the amount of turbulent flow of the first and second refrigerants 32, 34 within the interstage heat exchanger 38, which in turn maximizes the heat transfer from the condensing second refrigerant 34 to the evaporating first refrigerant 32. Other suitable types or configurations of heat exchangers are possible as well. The second refrigerant 34 exits the interstage heat exchanger 38 in liquid form and flows through the second expansion device 50, and then back to the evaporator 52, as the above-described process is repeated to cool the cabinet interior 18 of the freezer 10.

[0032] With continued reference to FIG. 2, in operation, the first refrigerant 32 flows through the first refrigeration stage 24. Specifically, the first refrigerant 32 receives heat energy from the second refrigerant 34 flowing through the interstage heat exchanger 38, as noted above, then exits the interstage heat exchanger 38 in gas form and towards the first compressor 40. From the first compressor 40, the compressed first refrigerant 32 flows into the condenser 42. The first refrigerant 32 in condenser 42 transfers heat to the surrounding environment 36 as the first refrigerant 32 condenses before flowing in liquid form into the first expansion device 44, where the first refrigerant 32 undergoes a pressure drop. From the first expansion device 44, the first refrigerant 32 flows back into the interstage heat exchanger 38, entering in liquid form.

[0033] With continued reference to FIG. 2, at least one of the first or second compressors 40, 48 of this embodiment of the refrigeration system 22 is a variable speed compressor. In a specific embodiment, the first and second compressors 40, 48 may have different maximum capacities. For example, and without limitation, the second compressor 48 may have a maximum capacity that is less than the maximum capacity of the first compressor 40. Alternatively, the maximum capacities of the first and second compressors 40, 48 may be substantially equal. Moreover, operation of the system 22 may be designed such that, in steady-state mode, one or both compressors 40, 48 operates at the maximum capacity or at less than its maximum capacity, which may be desirable, for example, to maximize the life expectancy of the compressors 40, 48. In other embodiments, the first and/or second compressors 40, 48 are fixed-speed compressors. To this end, the use of adjustable expansion in the refrigeration stages 24, 26 allows for operational variation of parameters to improve efficiency even with fixed-speed compressors.

[0034] As shown in FIG. 2, the refrigeration system 22 also includes an exemplary controller 54 that is operatively coupled to each of the first and second compressors 40, 48 for independently controlling each of the compressors 40, 48. While this embodiment illustrates a single controller 54, those of ordinary skill in the art will readily appreciate that system 22 may have any other number of controllers instead. An exemplary interface 56 is operatively coupled to the controller 54 to enable interaction with the controller 54 by a user. Such interaction may include, for example, choosing from among different modes of operation of system 22. The controller 54 is also operatively coupled to a plurality of sensors S.sub.1-S.sub.11 configured to sense different properties of the first and second refrigerants 32, 34 along the first and second fluid circuits 28, 30 of the first and/or second refrigeration stages 24, 26.

[0035] As shown in FIG. 2, the first fluid circuit 28 includes a suction pressure sensor S.sub.1 configured to measure an intake pressure generated by the first compressor 40 and a suction temperature sensor S.sub.2 configured to measure a temperature of the refrigerant 32 entering the first compressor 40. The first fluid circuit 28 further includes a liquid temperature sensor S.sub.3 operatively disposed in-line between the condenser 42 and the expansion device 44 that is configured to measure a temperature of the liquid refrigerant 32 leaving the condenser 42. The interstage heat exchanger 38 includes at least a temperature sensor S.sub.4 for measuring an operating temperature of the interstage heat exchanger 38. The temperature sensor S.sub.4 may be located on an exterior surface of the heat exchanger 38. For example, the temperature sensor S.sub.4 may be located on the exterior surface near the middle of the heat exchanger 38 or, alternatively, on a lower part of the exterior surface of the heat exchanger 38. The first fluid circuit also includes a temperature sensor S.sub.11 configured to measure a temperature of the refrigerant at an outlet of the heat exchanger 38.

[0036] The second fluid circuit 30 also includes a suction pressure sensor S.sub.5 configured to measure an intake pressure generated by the second compressor 48 and a suction temperature sensor S.sub.6 configured to measure a temperature of the refrigerant 34 entering the second compressor 48. The second fluid circuit 30 further includes a liquid temperature sensor S.sub.7 operatively disposed in-line between the interstage heat exchanger 38 and the expansion device 50 that is configured to measure a temperature of the refrigerant 32 leaving the interstage heat exchanger 38. The second fluid circuit also includes at least an evaporator inlet temperature sensor S.sub.8 and an evaporator outlet temperature sensor S.sub.9 configured to measure a temperature of the refrigerant 34 entering and leaving the evaporator 52, respectively. The cabinet interior 18 includes at least a cabinet temperature probe S.sub.10 configured to measure a temperature of the cabinet interior 18. The refrigeration system also includes an ambient air temperature sensor (not shown) for measuring the temperature of the ambient air surrounding the system 22 as well as various other sensors for detecting operational parameters of the freezer 10, such as a door 16 open sensor (not shown), for example. The sensors S.sub.1-S.sub.11 are configured to generate respective signals to the controller 54 that are indicative of selected operating conditions of the freezer 10, such that the controller 54 may, in turn, generate respective control signals for varying operation of the system 22 in response to the signals generated by the sensors, as will be described in further detail below.

[0037] Having described the detailed structural layout of the refrigeration system 22 used with embodiments of the freezer 10 of this invention, reference now turns to FIG. 3 which illustrates a specific embodiment allowing for optimal performance of the freezer 10. The cascade refrigeration system 22 is again shown, but with only the core equipment elements shown to simplify the illustration. It will be understood that additional equipment consistent with what is shown in FIG. 2 and described above would also typically be included in this embodiment.

[0038] With reference to FIG. 3, the refrigeration system 22 again includes the first refrigeration stage 24 including the first compressor 40, the condenser 42, the first expansion device 44, and the interstage heat exchanger 38 in sequence to define the first fluid circuit 28 through which the first refrigerant 32 flows. More particularly, the first fluid circuit 28 is defined by a first fluid line 66 for conveying refrigerant 32 from an outlet of the condenser 42 to a first inlet 68 of the interstage heat exchanger 38 and a first suction line 70 for conveying refrigerant 32 from a first outlet 72 of the interstage heat exchanger 38 to an inlet of the first compressor 40, as indicated by the directional arrows. A fluid line 71 is provided from the outlet of the first compressor 40 to an inlet of the condenser 42 for conveying refrigerant 32 back to the condenser 42. As shown, the first expansion device 44 is disposed along the first fluid line 66 and the first compressor 40 is disposed along the first suction line 70.

[0039] The refrigeration system 22 also includes the second refrigeration stage 26 having the second expansion device 50, the evaporator 52, the second compressor 48, and the interstage heat exchanger 38, in sequence, to define the second fluid circuit 30 through which the second refrigerant 34 flows. The second fluid circuit 30 is defined by a second fluid line 74 for conveying refrigerant 34 from a second outlet 76 of the interstage heat exchanger 38 to the evaporator 52 and a second suction line 78 for conveying refrigerant 34 from the evaporator 52 back to a second inlet 80 to the interstage heat exchanger 38, as indicated by the directional arrows. As shown, the second expansion device 50 is disposed along the second fluid line 74 and the second compressor 48 is disposed along the second suction line 78. The second fluid line 74 may temporarily branch into multiple fluid lines in parallel flow arrangement to accommodate for components of the expansion device 50, for reasons that will become clearer below.

[0040] To allow each of the heat exchanging elements (for example, condenser 42, interstage heat exchanger 38, and evaporator 52) to operate at respective optimized temperatures for the current operating conditions at the freezer 10, the first and second expansion devices 44, 50 are provided in this embodiment as adjustable expansion devices. In that regard, the first expansion device 44 is in the form of an electronic expansion valve (EEV) 58. The second expansion device 50 is defined by a first capillary tube 60 and a second capillary tube 62 in parallel flow arrangement with a valve 64 positioned upstream from the second capillary tube 62 to selectively control flow of the second refrigerant 34 through the second capillary tube 62. More particularly, the valve 64 is positioned so as to only control flow of the second refrigerant 34 through the second capillary tube 62. Thus, the second refrigerant 34 is free flowing from the interstage heat exchanger 38 through the first capillary tube 60 independent of the operational status of the valve 64 (for example, whether the valve 64 is opened or closed). Once the valve 64 is opened, the second refrigerant 34 flows through the second capillary tube 62 in addition to also flowing through the first capillary tube 60. To this end, while the second fluid line 74 branches into parallel flowing fluid lines to accommodate the first and the second capillary tubes 60, 62, the parallel flowing fluid lines rejoin to a single fluid line to direct a single flow of refrigerant 34 into the evaporator 52. The adjustable expansion provided by these elements in both refrigeration stages 24, 26 (in combination with setting the speed of the first and second compressors 40, 48 when such elements are variable speed compressors) helps set the temperature of refrigerant flowing through each of the heat exchanging elements in such a manner that can be controlled to be optimized for the current cabinet interior 18 setpoint temperature or other current operating conditions at the freezer 10, such as the ambient temperature, for example. In another embodiment, the parallel flowing fluid lines from the first and second capillary tubes 60, 62 each flow into the evaporator 52 without rejoining. A length of the second fluid line 74 from the second outlet 76 to the second capillary tube 62 can be greater than a length of the second fluid line 74 from the second capillary tube 62 to the evaporator 52. A length of the second fluid line 74 from the second outlet 76 to the first capillary tube 60 can be greater than a length of the second fluid line 74 from the first capillary tube 60 to the evaporator 52.

[0041] The electronic expansion valve 58 forming the first expansion device 44 in this embodiment may be any known commercial design capable of controlling the flow of refrigerant into the interstage heat exchanger 38. In that regard, the electronic expansion valve 58 can adjust the expansion and flow of refrigerant reliably at the operating temperature ranges of the first refrigerant 32 used in the first refrigeration stage 24 of the cascaded refrigeration system 22. By contrast, the second expansion device 50 in the second refrigeration stage 26 is provided by the two capillary tubes 60, 62 in parallel flow arrangement and the valve 64. Each capillary tube 60, 62 may have a same length and cross-sectional size, or different, so that an adjustable amount of refrigerant 34 expansion is provided by selectively flowing refrigerant 34 through one or both of the capillary tubes 60, 62 through operation of the valve 64. In this regard, the valve 64 is a solenoid valve capable of operating in the low temperature range of the second refrigeration stage 26.

[0042] By way of example, at a predetermined temperature setpoint, such as a warm temperature setpoint (for example, 50 C.), the valve 64 will be opened to increase the mass flow of refrigerant 34 through the evaporator 52. In this regard, while the flow of refrigerant 34 through both the first and second capillary tubes 60, 62 increases the mass flow rate of refrigerant 34 through the evaporator 52, causing an increase in evaporator 52 temperature, it also improves system efficiency and reduces temperature peak variation in the cabinet interior 18. To this end, the increased mass flow rate of refrigerant 34 through the evaporator 52 minimizes extreme cold spots within the cabinet interior 18 and reduces the temperature peak variation within the cabinet interior across different ambient conditions. The valve 64 may be similar to the Type EVU solenoid valves commercially available from Danfoss LLC (Baltimore, Maryland), for example. Operation of the valve 64 may be manual or in response to at least one operating condition of the refrigeration system 22. Thus, the valve 64 may be in a normally closed position and powered open once an appropriate signal is received.

[0043] Operation of the valve 64 may be based on the current operating conditions and instructions from the primary controller 54. In one embodiment, the controller 54 operates the valve 64 in response to an actual temperature of the cabinet interior 18 measured by the cabinet temperature probe S.sub.10. For example, the controller operates the valve 64 open if the temperature of the cabinet interior 18 measured by the cabinet temperature probe So is greater than 55 C. If the temperature of the cabinet interior 18 measured by the cabinet temperature probe S.sub.10 is lower than 55 C., the controller 54 operates the valve 64 closed. In another embodiment, the second stage valve 64 will be controlled by the setpoint at the beginning of the steady-state cycle. For example, the valve 64 is closed if the setpoint is lower than 60 C. and the valve 64 is opened if the setpoint is higher than 60 C. In another embodiment, the solenoid valve 64 is controlled by superheat of the evaporator 52 (for example, if the superheat of the evaporator 52 is greater than a threshold, the second stage valve 64 will be opened). To this end, further control of the temperature of the cabinet interior 18 may be accomplished by varying the speed of the compressor 48, for example. This adjustable expansion of refrigerants provided in at least the second refrigeration stage 26 sets the temperature of refrigerant 34 flowing through the evaporator 52 to minimize energy consumption as well as peak temperature fluctuation around the setpoint in the cabinet interior 18. These temperatures also optimize temperature pull down and recovery times by making those operations occur more quickly than would be the case with different operating temperatures of the refrigerant at the evaporator 52.

[0044] Both the switching valve 64 and the capillary tubes 60, 62 continue to operate reliably at all operating temperature ranges in the second refrigeration stage 26. For example, the range of cabinet temperatures and setpoints that typically occurs in a ULT freezer is about 20 C. to 90 C. Note that while setpoint adjustment is generally referred to as a setpoint temperature of the cabinet interior 18 in this description, the setpoint may also refer to setting the temperature of one or more heat exchangers within the freezer 10. Refrigeration systems in conventional ULT freezers are typically set up so that the operating temperatures of the heat exchanging elements are set for optimal performance in worst-case operating conditions (for example, optimally cooling to a 80 C. setpoint temperature, or lower, at an ambient outside temperature of 20 C., and being capable of reliably operating within an ambient temperature range of between 10 C. to 35 C.). To this end, typically one or both compressors and the expansion devices are tailored specifically for this one-use case or operating condition, which means whenever the ULT freezer operates at other operating conditions, the performance is not optimized because of the heat exchanging devices being at temperatures not tailored for maximum energy efficiency and heat transfer in those other operating conditions. However, the adjustable expansion provided by the electronic expansion valve 58 and by the capillary tubes 60, 62 alone, or in combination with variable speeds at the compressors 40, 48, enables more precise control options for setting the operational temperatures of the heat exchanging elements, including the condenser 42, the interstage heat exchanger 38, and the evaporator 52. Thus, an optimized temperature setting for each of these heat exchanging elements can be provided depending on the operating conditions measured by the various sensors in communication with the primary controller 54.

[0045] To further optimize performance and efficiency of the refrigeration system 22, the first and the second refrigeration stages 24, 26 can each include a heat exchanger 82, 84, respectively. Depending on the system configuration, the heat exchangers 82, 84 can be in the form of a liquid line to suction line heat exchanger (LLSLHX) or a capillary tube to suction line heat exchanger (CTSLHX). In the parallel capillary tube configuration described above with respect to FIG. 3, typically, the capillary tube that is not associated with the solenoid valve (for example, the first capillary tube 60) will include the heat exchanger in the form of a CTSLHX. When the expansion device does not include a capillary tube, the heat exchanger will be in the form of a LLSLHX.

[0046] With respect to the first refrigeration stage 24 shown in FIG. 3, the heat exchanger 82 is in the form of a LLSLHX. As an example, the heat exchanger 82 of the first refrigeration stage 24 forms part of the first fluid circuit 28 where a section 86 of the first fluid line 66 is placed in heat transferring communication with a section 88 of the first suction line 70 to transfer heat from the refrigerant 32 flowing through the first fluid line 66 to the refrigerant 32 flowing through the first suction line 70. As shown, the section 86 of the first fluid line 66 that forms part of the heat exchanger 82 is located upstream from the first expansion device 44 to subcool the hot refrigerant 32 before it enters the first expansion device 44 and ultimately the interstage heat exchanger 38. Subcooling of the refrigerant 32 improves operation of the first expansion device 44 and the interstage heat exchanger 38 and thus the efficiency of the system. The section 88 of the first suction line 70 that forms part of the heat exchanger 82 is located upstream from the first compressor 40 to heat the refrigerant 32 before it enters the first compressor 40. More particularly, heating of the refrigerant 32 improves the reliability of the system by vaporizing any droplets that accumulate in the first suction line 70.

[0047] With continued reference to FIG. 3, the heat exchanger 84 of the second refrigeration stage 26 is in the form of a CTSLHX. In that regard, the heat exchanger 84 forms part of the second fluid circuit 30 and is where a portion, or the entirety of, the first capillary tube 60 is placed in heat transferring communication with a section 90 of the second suction line 78 to transfer heat from the refrigerant 34 flowing through the first capillary tube 60 to the refrigerant 34 flowing through the second suction line 78. In some embodiments, second capillary tube 62 is isolated from the heat exchanger 84. A second capillary tube that is isolated from the heat exchanger 84 can increase energy efficiency of the system. For example, the refrigerant 34 flowing through the first capillary tube 60 cools the refrigerant flowing through the section 90 of the second suction line 78 while the refrigerant flowing through the second capillary tube 62 maintains its temperature as the refrigerant flows to the evaporator. In some embodiments, the heat exchanger 84 may be defined by a portion, or the entirety of, the first capillary tube 60 being placed in heat transferring communication with the section 90 of the second suction line 78 to transfer heat from the refrigerant 34 flowing through the first capillary tube 60 to the refrigerant 34 flowing through the second suction line 78 while the second capillary tube 62 is isolated from heat transferring communication with the section 90 of the second suction line 78. The section 90 of the second suction line 78 that forms part of the heat exchanger 84 is also located upstream from the second compressor 48 to heat the refrigerant 34 before it enters the compressor 48. Heating the refrigerant 34 before it enters the compressor 48 can maximize the amount of gas, rather than liquid, flows into the compressor. Increasing the amount of gas, rather than liquid, that flows into the compressor can improve the reliability of the compressor. This can also reduce or eliminate condensation on the compressor shell. In another embodiment, the heat exchanger 84 may be defined by a portion, or the entirety of, the second capillary tube 62 being placed in heat transferring communication with the section 90 of the second suction line 78 to transfer heat from the refrigerant 34 flowing through the second capillary tube 62 to the refrigerant 34 flowing through the second suction line 78. In yet another embodiment, the heat exchanger 84 may be a LLSLHX defined by a portion of the second fluid line 74 being placed in heat transferring communication with the section 90 of the second suction line 78. The second capillary tube 62 being in heat transferring communication with the section 90 of the second suction line 78 can allow for simpler manufacturing. For example, it can be simpler to have only the second capillary tube 62 in heat transferring communication than having both the first and second capillary tubes 60, 62. Although a capillary tube and a liquid line can each achieve the same heat transfer rate, the capillary tube can achieve the heat transfer rate over a shorter length of tubing than the liquid line. The capillary tube can provide a faster temperature pull down in the heat exchanger 84 than the liquid line.

[0048] To provide one example of operation of the refrigeration system 22 described above with respect to FIGS. 1-3, in a first operating state, the freezer 10 is tasked with setting the cabinet interior 18 to 86 C. when the ambient temperature surrounding the cabinet 12 is 20 C. In this state, the controller 54 operates the electronic expansion valve 58 in the first refrigeration stage 24 by actively controlling superheat of the refrigerant 32 at the interstage heat exchanger 38, and operates closed the second stage valve 64 to flow refrigerant 34 in the second refrigeration stage 26 through the first capillary tube 60 only. The second stage valve 64 controls the temperature of the evaporator 52 in response to the temperature of the cabinet interior 18 measured by the cabinet temperature probe S.sub.10. This adjustable amount of expansion of refrigerants provided in the first and second refrigeration stages 24, 26 sets the temperature of refrigerant 32, 34 flowing through each of the condenser 42, the interstage heat exchanger 38, and the evaporator 52, to minimize energy consumption as well as peak temperature fluctuation allowed around the setpoint in the cabinet interior 18. These temperatures also optimize temperature pull down and recovery times by making those operations occur more quickly than would be the case with different operating temperatures at the heat exchanging elements.

[0049] Now assume that the freezer 10 is to change modes to a second operating state and set the cabinet interior 18 to 50 C. with an ambient temperature reduced to 10 C. To account for these operational differences, the controller 54 would then adjust operation of the compressors 40, 48 (for example, variable speed compressors) and the first and second expansion devices 44, 50. Compared to the first operating state, the electronic expansion valve 58 will regulate to the same superheat setpoint while the second stage valve 64 is open. The adjustments to refrigerant expansion and/or compression changes the operating temperatures of the refrigerants 32, 34 delivered into the condenser 42, the interstage heat exchanger 38, and the evaporator 52. These new operating temperatures are tailored for more energy-efficient performance and heat transfer for the second operating state than if the temperatures were repeated from the first operating state. Furthermore, adjustable expansion is reliably enabled by selectively using the first and the second capillary tubes 60, 62 in the second refrigeration stage 26 to improve the performance of the freezer 10 in a plurality of different operating conditions.

[0050] In summary, the use of the adjustable refrigerant expansion in the freezer 10 according to the embodiments of this invention allows for a broader range of optimal performance operational setpoints (for example, 20 C. to 90 C. in one example). At any setpoint in this broad range, the freezer 10 functions to keep a consistently low evaporator 52 (and other heat exchanger) temperature profile non-uniformity, which is to say, minimized spikes over operational cycles. The peak variation of temperature in the cabinet interior 18 is also minimized thanks to the low levels of temperature non-uniformity achieved. As a result, energy consumption is improved or minimized at all the setpoints which improves operational stability and reliability of the freezer 10. Thus, the freezer 10 of the present invention provides several technical advantages and effects over conventional designs.

[0051] In one alternative embodiment of the refrigeration system 22 described above with respect to FIGS. 1-3, the second stage valve 64 may instead be an electronic expansion valve. In that regard, the controller 54 is configured to operate the electronic expansion valve between a fully closed and a fully opened position in the same manner as the second stage valve 64 described above. Additionally, the controller 54 is configured to operate the electronic expansion valve to precisely control the flow of the refrigerant 34 through the second capillary tube 62 to thereby control load in smaller increments. The controller 54 may be configured to operate the electronic expansion valve in this manner and in response to a temperature of the heat exchanger 84 measured by temperature sensor S.sub.4, such as during pull down, for example. Once at the steady operation, the controller is configured to precisely adjust the electronic expansion valve in response to a temperature difference between the evaporator inlet temperature measured by temperature sensor S.sub.8 and the evaporator outlet temperature measured by temperature sensor S.sub.9. To this end, the electronic expansion valve may be throttled to any position between full open and closed to accommodate for the needs of the system 22. The result is more precise control of refrigerant 34 flow and expansion in the second refrigeration stage 26, resulting in reduced energy consumption and accurate responses to load changes experienced by the freezer 10, for example. In this embodiment, the first refrigeration stage 24 may include the fluid line to suction line heat exchanger 82 where a section 86 of the first fluid line 66 is placed in heat transferring communication with a section 88 of the first suction line 70. Furthermore, the second refrigeration stage 26a may also include the capillary tube to suction line heat exchanger 84 where a portion, or the entirety of, the first capillary tube 60 is placed in heat transferring communication with a section 90 of the second suction line 78.

[0052] In another alternative embodiment of the refrigeration system 22 described above with respect to FIGS. 1-3, the second stage valve 64 and the second capillary tube 62 may be replaced with a single electronic expansion valve. In that regard, the controller 54 is configured to operate the electronic expansion valve between a fully closed and a fully opened position in the same manner as the second stage 64 described above. Additionally, the controller 54 is configured to operate the electronic expansion valve to precisely control the flow of the refrigerant 34 through the second capillary tube 62 to thereby control load in smaller increments. The controller 54 may be configured to operate the electronic expansion valve in this manner and in response to a temperature of the heat exchanger 84 measured by temperature sensor S.sub.4, such as during pull down, for example. Once at the steady operation, the controller is configured to precisely adjust the electronic expansion valve in response to a temperature difference between the evaporator inlet temperature measured by temperature sensor S.sub.8 and the evaporator outlet temperature measured by temperature sensor S.sub.9. To this end, the electronic expansion valve may be throttled to any position between full open and closed to accommodate for the needs of the system 22. The result is more precise control of refrigerant 34 flow and expansion in the second refrigeration stage 26, resulting in reduced energy consumption and accurate responses to load changes experienced by the freezer 10, for example. In this embodiment, the first refrigeration stage 24 may include the liquid line to suction line heat exchanger 82 where a section 86 of the first fluid line 66 is placed in heat transferring communication with a section 88 of the first suction line 70. Furthermore, the second refrigeration stage 26a may also include the capillary tube to suction line heat exchanger 84 where a portion, or the entirety of, the first capillary tube 60 is placed in heat transferring communication with a section 90 of the second suction line 78.

[0053] In another alternative embodiment of the refrigeration system 22 described above with respect to FIGS. 1-3, the second stage valve 64 and the second capillary tube 62 are replaced with a single electronic expansion valve and the first capillary tube 60 is also replaced with a single electronic expansion valve. In that regard, the two electronic expansion valves are in parallel flow arrangement. The controller 54 is configured to operate each electronic expansion valve to control evaporator 52 superheat. More particularly, the controller 54 is configured to operate each electronic expansion valve between a fully closed and a fully opened position in the same manner as the second stage valve 64 described above. Additionally, the controller 54 may be configured to operate one or both electronic expansion valves in this manner and in response to a temperature of the heat exchanger 84 measured by temperature sensor S.sub.4, such as during pull down, for example. Once at the steady operation, the controller is configured to precisely adjust each electronic expansion valve in response to a temperature difference between the evaporator inlet temperature measured by temperature sensor S.sub.8 and the evaporator outlet temperature measured by temperature sensor S.sub.9. To this end, each electronic expansion valve may be throttled to any position between full open and closed to accommodate for the needs of the system 22. The result is more precise control of refrigerant 34 flow and expansion in the second refrigeration stage 26, resulting in reduced energy consumption and accurate responses to load changes experienced by the freezer 10, for example. In this embodiment, the first refrigeration stage 24 may include the fluid line to suction line heat exchanger 82 where a section 86 of the first fluid line 66 is placed in heat transferring communication with a section 88 of the first suction line 70. Furthermore, the second refrigeration stage 26a may also include a fluid line to suction line heat exchanger where a section second fluid line 74 is placed in heat transferring communication with a section of the second suction line 78.

[0054] With reference to FIG. 4, wherein like numerals represent like features, details of a second embodiment of the refrigeration system 22a that may be used with the freezer 10 are shown in accordance with another embodiment of the present invention. The primary differences between the refrigeration system 22a of this embodiment and the refrigeration system 22 of the previously described embodiment is the configuration of the first refrigeration stage 24a. In that regard, the refrigeration system 22a includes first and second cascaded refrigeration stages 24a, 26a in a similar configuration as the previously described embodiment, with many of the same elements (first compressor 40a, condenser 42a, interstage heat exchanger 38a, second compressor 48a, and evaporator 52a) carrying similar reference numbers without further description herein. The second fluid circuit 30a is similar to the second fluid circuit 30 described above with respect to FIGS. 1-3, and includes the second expansion device 50a defined by a first capillary tube 60a and a second capillary tube 62a in parallel flow arrangement with a valve 64a positioned upstream from the second capillary tube 62a to selectively control flow of the second refrigerant 34a through the second capillary tube 62a. Like the embodiment described above with respect to FIG. 3, the valve 64a is positioned so as to only control flow of the second refrigerant 34a through the second capillary tube 62a. Thus, the second refrigerant 34a is free flowing from the interstage heat exchanger 38a through the first capillary tube 60a independent of the operational status of the valve 64a (for example, whether the valve 64 is opened or closed). The second refrigeration stage 26a also includes the heat exchanger 84a which is in the form of a CTSLHX. In one embodiment, the heat exchanger 84a may be a LLSLHX defined by a portion of the second fluid line 74a being placed in heat transferring communication with the section 90a of the second suction line 78a.

[0055] The first fluid circuit 28a is defined by a first fluid line 66a for conveying refrigerant 32a from the condenser 42a to of the first inlet 68a of the interstage heat exchanger 38a and a first suction line 70a for conveying refrigerant 32a from the first outlet 72a of the interstage heat exchanger 38a to an inlet of the first compressor 40a, as indicated by the directional arrows. A fluid line 71a is provided from the outlet of the first compressor 40a to an inlet of the condenser 42a for conveying refrigerant 32a back to the condenser 42a. As shown, the first expansion device 44a is disposed along the first fluid line 66a and the second compressor 40a is disposed along the first suction line 70a. More particularly, the first fluid line 66a temporarily branches into multiple fluid lines in parallel flow arrangement to accommodate for components of the expansion device 44a.

[0056] The parallel flow arrangement of the first fluid line 66a may be similar to that of the second fluid line 74a of the second fluid circuit 30a. In that regard, the first expansion device 44a is defined by a first capillary tube 92 and a second capillary tube 94 in parallel flow arrangement with a valve 96 positioned upstream from the second capillary tube 94 to selectively control flow of the first refrigerant 32a through the second capillary tube 94. More particularly, the valve 96 is positioned so as to only control flow of the first refrigerant 32a through the second capillary tube 94. As a result, the first refrigerant 32a is free flowing from the condenser 42a through the first capillary tube 92 independent of the operational status of the valve 96 (for example, whether the valve 96 is opened or closed). Once the valve 96 is opened, the first refrigerant 32a flows through the second capillary tube 94 in addition to also flowing through the first capillary tube 92. To this end, while the first fluid line 66a branches into parallel flowing fluid lines to accommodate the first and the second capillary tubes 92, 94, the parallel flowing fluid lines rejoin to a single fluid line 66a to direct a single flow of refrigerant 32a into the first inlet 68a of the interstage heat exchanger 38a.

[0057] The adjustable expansion provided by the parallel capillary tube arrangement in both refrigeration stages 24a, 26a (in combination with setting the speed of the first and second compressors 40a, 48a when such elements are variable speed compressors) helps set the temperature of refrigerant 32a, 34a flowing through each of the heat exchanging elements (for example, condenser 42a, interstage heat exchanger 38a, and evaporator 52a) in such a manner that can be controlled to be optimized for the current cabinet interior 18 setpoint temperature or other current operating conditions at the freezer 10.

[0058] To further optimize performance, efficiency, and pull down time for the refrigeration system 22a, the first refrigeration stage 24a also includes a heat exchanger 82a, which is in the form of a CTSLHX. In that regard, the heat exchanger 82a forms part of the first fluid circuit 28a and is where a portion, or the entirety of, the first capillary tube 92 is placed in heat transferring communication with a section 98 of the first suction line 70a to transfer heat from the refrigerant 32a flowing through the first capillary tube 92 to the refrigerant 32a flowing through the first suction line 70a. The section 98 of the first suction line 70a that forms part of the heat exchanger 82a is located upstream from the first compressor 40a to heat the refrigerant 32a before it enters the compressor 40a to improve cycle efficiency. In one embodiment, the heat exchanger 82a may be a LLSLHX defined by a portion of the first fluid line 66a being placed in heat transferring communication with the section 98a of the first suction line 70a.

[0059] While the present invention has been illustrated by a description several exemplary embodiments and while these embodiments have been described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the general inventive concept.